Improved flaw detection apparatus using specially located hall detector elements

ABSTRACT

A pair of Hall generator elements are symmetrically mounted in the same magnetic plane closely adjacent a material surface of substantially circular cross section, one Hall generator element being mounted on each side of a null plane, to detect the variations in leakage flux emanating from the material surface as the material is spirally rotated past the Hall generator elements. The Hall generator elements are symmetrically mounted on an axis intersecting a line parallel to the longitudinal axis of the material by approximately 5* and a control current provided to each Hall generator element of the same magnitude but different polarity. The Hall generator elements are, alternately, mounted astride the null plane to be substantially nearly aligned with the null plane and the control current to each Hall generator element is of the same magnitude and polarity.

United States Patent [72] Inventor Takayuki Kanbayashi 552 Niina, Minoo,Japan [21] Appl. No. 830,624 [22] Filed June 5, 1969 [45] Patented May18, 1971 [32] Priority Dec. 4, 1965 [33} Japan [31] 40/74740Continuation-impart of application Ser. No. 598,457, Dec. 1, 1966, nowabandoned.

[54] IMPROVED FLAW DETECTION APPARATUS USING SPECIALLY LOCATED HALLDETECTOR ELEMENTS 7 Claims, 14 Drawing Figs. [52] US. Cl 324/37 [51]G0lr 33/12 [50] Field of Search 324/37, 40, 45, 46

[5 6] References Cited UNITED STATES PATENTS 1,998,952 4/1935 Edgar eta1 324/37 2,219,885 10/1940 Barnes et a1. 324/37 2,746,012 5/1956 Price324/37 3,202,914 8/1965 Deem et a] 324/37 3,484,682 12/1969 Wood 324/37FOREIGN PATENTS 822,210 10/ l959 Great Britain 324/45 950,696 2/1964Great Britain 324/ 37 ABSTRACT: A pair of Hall generator elements aresymmetrically mounted in the same magnetic plane closely adjacent amaterial surface of substantially circular cross section, one Hallgenerator element being mounted on each side of a null plane, to detectthe variations in leakage flux emanating from the material surface asthe material is spirally rotated past the Hall generator elements. TheHall generator elements are symmetrically mounted on an axisintersecting a line parallel to the longitudinal axis of the material byapproximately 5 and a control current provided to each Hall generatorelement of the same magnitude but different polarity. The Hall generatorelements are, alternately, mounted astride the null plane to besubstantially nearly aligned with the null plane and the control currentto each Hall generator element is of the same magnitude and polarity.

- wall/III Patented May 18, 1971 3,579,099

4 Sheets-Sheet l INVENTOR 00:12A Cml 5 L4) ATTY )R f Patented May 18,1971 3,579,099

4 Sheets-Sheet 4 MIL... C1124, w-u, -v w liow ATTORNEYS IMPROVED IFLAWDIETE'IITIUN AIPIPARATUS USING S PECIIALILY LGECATIED HALL DETECTORELEMENTS This is a continuation-in-part application of my copendingapplication Ser. No. 598,457, filed Dec. 1, 1966, and now abandoned.

This invention relates to magnetic inspecting apparatus.

Magnetic inspection requires that a magnetic leakage flux from a flaw ina magnetized material to be tested be detected by some means. Today, asthe nondestructive inspection of, for example a bar, relies mostly on afluorescent magnetic particle inspection method, the testing speed isslow and its is very difficult to estimate the flaw depth. Therefore,the development of an automatic inspecting method of sufficiently hightesting efficiency replacing the magnetic particle inspection method isdesired. The present inventor has developed an improved automaticapparatus for magnetic inspection in which a Hall generator is used asthe detecting means and which is disclosed herein.

The magnetic inspecting method is nondestructive and detects anydiscontinuity present in a magnetic material. Its principle is based onthe known fact that, if a magnetic material is magnetized, a magneticflux will leak from a discontinuity or flaw in the material and the flawcan be detected by detecting the magnetic leakage flux. The magnitude ofthe magnetic leakage flux will be the largest where the discontinuity ispresent on the surface of the test material. The more the discontinuitylies below the surface the magnetic leakage flux will be correspondinglysmaller. Therefore, the discontinuities that can be detected by magneticinspection are limited to those present on the surface or the immediatesubsurface of the test material. In spite of such a disadvantage, themagnetic inspecting method has been extensively used because theapparatus is simple and no complicated operation is required.

The objects to be tested by the magnetic inspecting method have beenmostly steel coatings and forgings. However, generally, most of suchcast and rolled materials are so geometrically complicated that it hasbeen difficult to automatically test them. Consequently, the variousautomatic inspecting apparatus utilized today test material of suchsimple form as a steel pipe or a round bar.

Naturally the shapes of flaws vary. The magnitude of the magneticleakage flux from the flaw varies so much with its shape (such as thewidth, depth, direction and edge sharpness) that it is almost impossibleto theoretically estimate the magnitude of the magnetic leakage flux.However, in practice, the problem of the quantitative determination inautomatic inspection is whether the flaw to be inspected has a depthlarger than an allowable depth limit. For example, such comparativelyshallow cracks as 0.3 mm. present a problem. In such a flaw, it has beenexperimentally identified that the main influencing factor on themagnetic leakage flux is the flaw depth.

The magnetic inspecting methods practiced today may be classified asfollows according to the detectors used for detecting the magieticleakage flux:

1. Magnetic particle method.

2. Magnetic recording method.

3. Searching coil method.

4. a Hall generator method.

In the magietic particle method, it is possible to detect fine flaws asfine magnetic particles are used, but it is not possible to estimate theflaw depth and the testing speed is slow.

In the magnetic recording method, wherein a recording tape made of asynthetic resin on which magnetic particles are deposited is used as adetector, the depth of the flaw can be estimated and the flaw detectingsensitivity is high but, since it is desired to bring the magnetic tapeinto close contact with the test material, such a method is not adaptedto the detection of flaws in semifinished products having rough surfacessuch as, for example, billets.

In the searching coil inspection method, the voltage generated from thesearching coil is proportional to the varia tion with time of themagnitude of the magnetic flux interlinked with the coil, that is,proportional to the differential value of the magnitude of the magneticinterlinked flux with time and the number of turns of the coil. In orderto increase the flaw detecting sensitivity, the number of turns of thecoil must be increased. There are certain defects requiring that thesize of the detector must be large. Also, the detecting voltage varieswith the moving velocity of the test material or the moving velocity ofthe searching coil. All these factors influence the testing precision.

The inspection method using a Hall generator, as described in detailhereinafter, has the advantages that, if the control current for theHall generator is kept constant, the generated voltage will beproportional to the magnitude of the magnetic flux which penetratesperpendicularly to the largest surface of the Hall generator and willnot be influenced by the moving velocity of the Hall generator or themoving velocity of the test material. Furthermore, the magnetic leakageflux will be able to be detected without contact with the test material.

In the automatic magnetic inspection of the present invention, assemifinished products especially bars or round material having roughsurfaces are to be inspected, Hall generators are effectively used asdetectors for the above-mentioned reasons.

In the accompanying drawings:

FIG. 1 illustrates the Hall effect on a semiconductor placed in amagnetic field:

FIG. 2A shows a Hall generator placed in a magnetic field and thedistribution of magnetic flux;

FIG. 28 illustrates the distortion of the magnetic flux pattern by aflaw in the material;

FIG. 3A is a plan view showing an arrangement of magnetic poles, a testmaterial and the Hall generators in a magnetic inspecting apparatus;

FIG. 3B is a side view of FIG. 3A;

FIG. 3C is an elevation view of FIG. 3A;

FIG. 4A is an elevation view of an embodiment of the apparatus formounting the detecting apparatus of the present invention;

FIG. 4B is a side view of FIG. 4A;

FIG. 5 is a combination circuit diagram and block diagram of theelectronic circuit for magnetic inspecting apparatus according to thepresent invention;

FIG. 6 illustrates an arrangement of two Hall generators in a magneticfield;

FIG. 7A shows another arrangement of two Hall generators in a magneticfield and the interconnection of the respective Hall generators;

FIG. 7B illustrates still another arrangement and interconnection of therespective Hall generators;

FIG. 8 illustrates the relationship between the magnetic flux component2 in the direction of the Z axis detected by each of two Hall generatorsin a given detector arrangement; and

FIG. 9 is a view showing an example of an inspection record obtainedwith a magnetic inspecting apparatus of the present invention.

An object of the present invention is to provide an apparatus forautomatically detecting very small flaws present in test materials byusing Hall generators.

A further object of the present invention is to provide an apparatus fordetecting flaws correctly without influence by any magnetic fluctuationeven when the test material is eccentric or noncircular.

The Hall effect is a phenomenon of a galvanomagnetic effect producedwhen a semiconductor is placed in a magnetic field. It is phenomenoncaused by a fl-ow of a carrier in a conductor placed so as to intersectat right angles with a magnetic field as shown in FIG. 1 whereby anelectromotive force will be generated in a direction at right angles toboth the magnetic field and the flow of the carrier. It was discoveredby E. H. Hall in 1879.

Now, if, as shown in FIG. 1, a current is made to flow in the directionof the X axis and a magnetic field is applied in the direction of the Zaxis at right angles thereto, the carrier will receive a Lorentz forceof e'VB in the direction of the Y axis. e is the carrier charge, V isthe carrier velocity and B is the flux density. As a result, the carrierwill be moved to the side A in the drawing to form a space charge.Therefore, electric field Ey will be induced in the direction of the Yaxis by these charges and will be balanced with the force by themagnetic field B acting on the carrier. Now, in the case of an N- type(in which the carrier is an electron and its charge is e). the forceacting on the electron by the magnetic field will be e-VB and the forceby the electric field to be balanced with it will be r Therefore, fromboth formulas, the electric field E will be E =V-B (3) If the currentdensity is i and the density of the carrier (electron) is n,

i=n(e-V) V=i/ne (4) Therefore, if the formula (4) is substituted in theformula (3),

Now, if

the formula (5) will be E,;=R-i.B 7

This R is a Hall constant.

(In the P-type, R=

Now, ifa current I in in ampere and a flux density B in gauss are givento the Hall generator, the Hall electromotive force V will be P v I 1538 wherein b is the width of the Hall generator and d is its thickness.

If a dimension in cm. is given to d,

V =R X 10 (volts) wherein R is in cm."/coulomb.

In order to increase the Hall voltage V it is desirable to make theelectron density n and the thickness d in the Hall generator small, toselect a material of a large mobility i, and to use a form of a surfacearea as wide as possible. The materials used today for Hall generatorsare germanium, silicon, indium antimony (InSb), indium arsenic (InAs),etc. The p. values of lnSb and InAs are large.

As it requires about 10 to 10 seconds for the Hall electromotive forceto be excited, the upper limit of the usable frequency of the Hallgenerator will be substantially 10 to 10 cycles/sec. However, as amatter of fact, in case the above-mentioned semiconductor is placed inan alternating magnetic field, eddy currents will be induced in it andthe Hall electromotive force will vary with the frequency. Therefore,the frequency range for obtaining uniform characteristics is though tobe 0 to 10 cycles/sec.

An automatic inspecting apparatus using the method of a low-frequencymagnetization and the novel arrangement of Hall generator which has beendeveloped by the present inventor will now be described.

In order to inspect an object using a magnetic inspecting method, it isnecessary that the test material should be properly magnetized. Thereare several methods to magnetize an object. One is the direct currentmethod in which a direct current is directly passed through the testmaterial and another is the method in which the test material ismagnetized by being held between magnetic poles. The direct currentmethod is used generally in a residual magnetic inspection method. Ithas a disadvantage that, in order to magnetize a test materialsufficiently to inspect it, a direct current of several hundred toseveral thousand amperes is required. In a permanent magnet method, noelectric source is required by the generated magnetic field is, ofcourse, a direct current magnetic field, resulting in a force attractingthe test material to the magnetic pole and the automatic test materialfeeding apparatus (such as, for example, a conveyor roller) will beundesirably magnetically braked. However, if such a defect is acceptedor due consideration is given to it, the permanent magnet can be used asthe magnetizing apparatus. If an alternating current electromagnet isused, the aforementioned problem is obviated. There is a low frequencymethod using a line frequency and a high frequency magnetizing method.With a line frequency of 50 to 60 cycles/see, no special current sourceequipment is required, but is it is possible that, if the testing speedis high, the test accuracy will not be sufficient. When the magnetizingfrequency us high, many eddy currents will be generated within the testmaterial and will influence the inspection precision when using amagnetic flux leakage detection method, in accordance with theinvention, which is essentially different from a flowdetecting apparatuswhich senses the effects of eddy currents. However, for these reasons, alow-frequency electromagnet is used. The apparatus of the presentinvention detects a leakage flux so that external magnetic fields,except the leakage flux, should not be detected. Furthermore, even ifundesirable magnetic fields are detected, they must be excluded from theoutput of the detection apparatus. For this purpose, it is necessary togive due consideration to the magnetic pole structure of theelectromagnet, the relative position between the material to be testedand the electromagnet and the position of the detector. In order todecrease the flux leakage directly from the magnetic poles it ispreferable to make the distance between the magnetic poles wide within arange in which the magnetization of the material is to be tested so thatthe magnetization at the point where Hall generator is set is notweakened.

It is necessary to prevent other external magnetic flux from passingthrough the Hall generator and, even if such undesirable magnetic fluxpasses through the Hall generator, to make such flux pass parallel withthe plane of the Hall generator so as not to generate an electromotiveforce. Further, although it is ideal to have all the magnetic flux fromthe magnetic poles enter into the material to be tested, in practicethere is stray magnetic flux from the poles which should not bedetected. Therefore, a preferred arrangement is to interpose the material 4 to be tested with a substantially U-shaped electromagnet and to setthe Hall generators at the center between the magnetic poles I, 2 of theU-shaped electromagnet, as shown in FIG. 28. It is preferred to have thesurface of the Hall generator close to the surface of the material to betested. In this case the distance between these surfaces is generallyO.l--2.00

The Hall generators are arranged in parallel with the surface of a bar,whereby the magnetic flux leaking from the flaw meters the Hallgenerator at an approximately right angle, thus providing an intensiveoutput, unaffected by the other magnetic flux lines which are parallelto each other and parallel to the Hall generator (FIG. 28) so as not tocause a voltage to be generated. Thus, the magnetic field created by theflaw provides the input flux to the Hall generator and in order tofurther eliminate detection of unwanted and stray magnetic flux thepresent invention employs a set of two Hall generators. The example ofthe abovementioned electromagnet is as shown in FIGS. 2A and 3A-3Cwherein 1600 ampere turns are adopted for the inspection of round stockhaving a diameter of 50 mm.

In the apparatus of the present invention, in order to eliminate theabove-mentioned defect, two Hall generators HG and HG are arranged inthe same plane on null plane of the magnetizing fields of the magneticpoles l and 2 so as to be very close to a test material i and thevoltages commonly provided by the respective Hall generators may becancelled with each other by connecting their outputs differentially tothe indicating apparatus. When a flaw is present under the Hallgenerator HG,, the difference |V,V between the Hall voltage V,, of theHall generator HG and the Hall voltage V of the Hall generator HG willbe of a magnitude proportional to the magnetic leakage flux from theflaw. Thereby, the signalto-noise ratio is elevated and it is possibleto automatically detect fine flaws.

The Hall generators to be used for the magnetic flux inspection are ofan indium arsenic system having an outside diameter of 3.7 mm. As shownin FIG. 4, two Hall generators are set at a spacing of 8 mm. These Hallgenerators operate on a DC control current of I80 ma. With respect toFIGS. 4A and 4B, the two generators HG and HG are incorporatedrespectively in detector mountings 21 and 22 and are fitted to amechanical stage 24 suspended from a nonmagnetic support 23 so that tworacks 27 provided on the mechanical stage may be slightly movedvertically by screws 25 and 26 and the right and left positions may beadjusted with a screw 28. By means of this adjusting device, the twoHall generators can be adjusted so as to be placed in equivalentmagnetic flux positions between the magnetic field poles of theelectromagnet. If the magnetic flux lines are equal to each other in thepositions where the two Hall generators are placed, little noise will beproduced.

The magnetic leakage flux from a flaw in the magnetized test material 4will be detected by the Hall generators HG and HG,, as shown in FIG. 2B,and will be converted to a voltage which is fed to an electronicprocessing circuit. This circuit is shown in FIG. 5. In FIG. 5 the Hallgenerators HG and HG, are provided with DC control current sources 5 and6, current controllers 7 and 8 and ammeters 9 and 10, respectively, andthe outputs of the Hall generators are wired to two preamplifiers 11 and12 of the same characteristics, respectively, and may be fed to anoutput transformer 14 through a balance-regulating circuit 13.

The control currents may also be supplied to HG and HG from one source.Further, as in FIG. 7, a single differential amplifier 20 may besubstituted for the two preamplifiers 11 and 12. In such case, it isnecessary that the output terminals of the Hall generators should bedifferentially wired as in FIG. 7. The output of the output transformer14 will pass through a variable frequency low-pass filter 15 and will beamplified in a main amplifier 16. After amplification, the output willbe rectified by rectifier l8, amplified by DC amplifier l9, and will berecorded in a recording device (not shown), such as an oscil' lographrecorder. The output signal of the main amplifier 16 is also supplied toa synchroscope 17 for monitoring flaw detection. The output of thedirect current amplifier 19 is used, though not illustrated, as an inputsignal for an automatic flawmarking device, warning device, automatictest material rejecting device and product quality classifying device asrequired.

For the successful use of this automatic inspecting method using Hallgenerators the most important consideration is the arrangement of thetwo Hall generators HG and H6 In the apparatus of the present invention,for the abovementioned reasons, two detectors are used and their outputsare differentially wired. However, in order that the outputs of thedetectors may be perfectly differential and the noise output may bezero, the magnitudes of the respectively detected external and strayflux must be equal to each other in the place where the two generatorsare positioned. That is to say, as shown in FIG. 6, if the angle 0,formed by the longitudinal axis of the test material and the lineconnecting the center of the detecting elements, is not zero, the noiseoutputs of the two detecting elements will not be zero. Only when themagnetic center x between magnetic poles l and 2 coincides with the linePQ connecting the center P and Q of the detecting elemerits will thenoise output be cancelled. If, because of the variation of the shape ofthe test material, the x axis moves and the magnetic center 0 betweenboth magnetic poles moves, even if the outputs of the detecting elementsare connected differentially, the noise will not be able to be perfectlycancelled. In inspecting such nonuniform and irregularly shaped testmaterial, it is advantageous to make the angle 6 zero. However, if themagnetic leakage flux from a flaw is detected simultaneously by bothdetectors, as for example with a long flaw lying in the axial direction,the output signal will become zero. Therefore, the angle 0 must not bezero for the detection of long flaws. Therefore, an angle ofapproximately 5 is used in detecting flaws.

Practical arrangements of Hall generators (detectors) are shown in FIGS.7A and B. In FIG. 7A, in order to make it possible to detect long flawsin test materials spirally rotated, two Hall generators are arranged inthe same plane in positions symmetrical to each other with respect tothe magnetic center axis x between the magnetic poles 1 and 2 of themagnetizing electromagnet. The null plane may be defined as that planetransverse to the field direction and parallel to the Ion gitudinal axisof the material along which the magnetic flux is zero. The DC controlcurrent supplied to one Hall generator is the same in magnitude butreversed in polarity to the control current supplied to the other Hallgenerator and the generated noise voltage will be of the same phase inthe two Hall generators. As a result, if the outputs of both Hallgenerators. As a result, if the outputs of both Hall generators aredifferentially amplified with differential amplifier 20, the noisevoltage will be cancelled and only the flaw-detecting voltage willappear as an output. However, the disadvantage of the system is that, ifthe test material 4 has an eccentric or noncircular cross section, a setof Hall generators must always be arranged in positions symmetrical toeach other with respect to the null plane between the magnetic poles.Now, if the positions of both Hall generators become magneticallyasymmetrical due to the eccentricity or other abnormality of the testmaterial, Hall generators HG and HG will provide different outputs. FIG.8 shows the variation of the magnetic flux component (b: in thedirection of the Z axis detected by the Hall generators HG and HG forvariations in the null plane position from the null plane a between bothmagnetic poles on the X axis to a point b. a is a null plane point for atest material of a circular cross section and b is a null plane pointfor a test material with an eccentricity to the right of center. If thecontrol current I is constant, the output voltage V of the Hallgenerator will be VII] KGB l n) and l VH2 I i l a),

the differential voltage V will be V m VH2 KKAIDI l a) Thus, if thepresent arrangement. is used when the null plane fluctuates onlyslightly, the influences of the magnetic fluctuation on both Hallgenerators will be added together and the noise voltage appearing in thedifferential output Will increase with increasing fluctuation.

In the system in FIG. 78, DC control currents of the same polarity andmagnitude are supplied to two Hall generators in a set with the centersof the two Hall generators slightly nonsymmetrical with respect the nullplane. In this arrangement, as the Hall generators can be considered tobe arranged substantially on a straight line in the axial direction (thedirection x in FIG. 7B) of the test material 4, the influence of themagnetic fluctuation caused by the eccentricity or the like of the testmaterial will be received to substantially the same degree by the twoHall generators and will not appear in the differential output. Further,if the Hall generator outputs are differentially amplified and if thetest material is propelled with spiral rotation, it will be possible todetect at substantially the same sensitivity not only partial flaws inthe test material but also flaws present over the entire length of thetest material.

The situation where the null plane position has changed from the point ato the point b is considered in the same manner as is mentioned above.Now, when the null plane position is at the point a,

VH1 KID and wherein K is a constant. As the control currents of the Hallgenerators HG and H6 are of the same phase, the signs of K of both willbe the same. Therefore, the differential output will be V=VH1 VH2=2KIThus a noise voltage will be generated. However, by arranging the Hallgenerators HG, and HG very close to the null plane position a,approaches zero and the noise is reduced so as not to be a problem inpractice. On the other hand, when the null plane position moves from thepoint a to the point b, as the respective Hall voltages are VH1 w i i)and (18) VH2 w the differential outputs V of both will be V VH1- VH2Kalb M01 A92) Therefore, according to the present arrangement, theinfluence of the magnetic fluctuation will appear as a difference valueand will be therefore much smaller than in the arrangement in FIG. 7A.In fact, the inspection of a round bar of 50 mm. diameter and 4 mm.eccentricity using the present arrangement resulted in the generation ofa periodic noise voltage of negligible value.

FIG. 9 illustrates the results of an inspection of a 50 mm. diameter barwith the apparatus of the present invention and shows detected flawscorresponding to microscopic photographs having a magnification of 30times.

The inspecting conditions were:

Air gap between the Hall generator and the test material: 0.3 mm.

Distance between the centers of the Hall generators: 8 mm.

Test material r.p.m.: 10 rpm.

Rotating feeding pitch of the test material: 5 mm./revolutron.

Gain of the main amplifier: 30 db.

Chart velocity: 1 mm./sec.

Magnetizing voltage, frequency and ampere l00v.60l ,560 ampere turns.

Now, in inspecting a round bar spirally at a pitch of 5 mm./revolution,in order to make the testing speed more than 5m./min., the number ofrevolutions of the test material must turns:

be made more than 100 rpm. As described above, if the test material isrotated at more than rpm, at a magnetizing frequency of 60 c/s, thereproductivity of the measurement will be poor. In order to determinethe frequency of the magnetizing current to be used in a case where thetest material rotates at a high speed, the frequency of the flaw signalmust be determined. Now, if it is assumed that the magnetic leakage fluxfrom a flaw is produced from one end of the flaw after entering theother end, that is, the width W in mm. of the flaw is equal to thewavelength of the flaw signal, and the magnetic leakage flux is to bedetected with two Hall generators, the frequency f in cycles/sec. of theflaw-detecting signal will be 65.5 cycles/sec. from the equation:

wherein r is the radius of the test material, b is the width of the Hallgenerator and RPS is a frequency per second.

It is desirable that the relation between the frequency fe of themagnetizing current and the frequency fs of the flaw detecting signal issuch that the magnetizing current should always have a momentary maximumvalue at least once in each half-cycle of the flaw signal. Therefore, itis desirable that fe 2 fs or, from a consideration of reproducibility,fe 3 fs. Therefore, in order that the test material may be inspected ata high degree of reproducibility at rotational speeds of more than 100rpm, the frequency of the magnetizing current must be higher than 200cycles/sec. However, when the frequency is more than 1,000 cycles/sec,because of the skin effect, there is an increase of noise due to thesurface roughness of the test material and it will not be desirable touse such a high-frequency current source.

Therefore, the present inventor has obtained an inspection precisionhigh enough for field tests by using a magnetizing frequency of 560cycles/sec. at a high speed of inspection and with a rotating speed ofrpm. by using the apparatus illustrated in FIG. 4. Briefly, as set forthabove, the use of eddy currents produces noise in the search coil orHall detector which is undesirable and reduces the applicability of suchprior art inspection techniques. Consequently, the apparatus of thepresent invention provides means for magnetizing a material so as toestablish therein a leakage flux which is created by the flaws withinthe material itself. Furthermore, the apparatus requires a pair of Hallgenerator elements for detecting the leakage flux emanating from a flawon the surface or within the material. These Hall generator elements aresymmetrically mounted with respect to the null plane which is induced inthe material by the magnetizing means. This enables the Hall generatorsto be located with respect to the null plane so as to cancel out thenoise voltages and thereby provide a more accurate output signal throughthe differentially connected detecting means to eliminate the noise andenhance the signal-to-noise ratio.

Iclaim:

1. Magnetic detection apparatus for detecting flaws in a material,comprising;

means for magnetizing the surface of a material of substantiallycircular cross section with an alternating magnetic flux, said fluxbeing directed transverse to the longitudinal axis of the material,means for spirally rotating the material along its longitudinal axis toexpose the entire surface of the material to the magnetizing means, saidmeans for magnetizing the surface causing said magnetic field to have azero component in a null plane transverse to said field direction andparallel to the longitudinal axis of the material,

a pair of Hall generator elements for detecting the leakage fluxemanating from a flaw in said material, said Hall generator elementslying adjacent the material surface in a plane parallel to the directionof the magnetic field with the centers of said Hall elements lying on anaxis which intersects a line parallel to the longitudinal axis of thematerial at an angle of substantially 5, means for adjusting the Hallelements in the direction of the magnetic field so that they aresymmetrically mounted one on each side of the null plane,

means for detecting the signal outputs from each of said Hall generatingelements being differentially connected to the detecting means toeliminate noise and enhance the signal-to-noise ratio.

2. Magnetic detection apparatus according to claim further comprisingmeans for applying a DC bias current to the Hall generator elements andwherein the DC bias current is of the same magnitude by but differentpolarity for each Hall generator.

3. Magnetic detection apparatus according to claim 1 further comprisingmeans for mounting each of the Hall generator elements with respect tothe material surface to be independently movable in each of threemutually perpendicular directions.

4. Magnetic detection apparatus according to claim 3 wherein said meansfor movably mounting said Hall generator elements includes a supportstructure, means for retaining each of said a Hall generator elements,said means for retaining being movable in a plane normal to the surfaceof said material, means for moving said retaining means in said normalplane, means for adjusting said means for retaining in a plane parallelto said longitudinal axis of said material, and means for moving saidmeans for retaining in a plane transverse to said longitudinal axis.

5. Apparatus according to claim 4 wherein said support structure furtherincludes spaced parallelly extending toothed racks, said means formoving said retaining means in said normal plane including rotatablemeans engaging said toothed racks, said means for retaining are mountedon threaded support members, said threaded support members are rotatableto provide said adjustment, and said means for moving said means forretaining in said transverse plane including threaded means engagingsaid support structure whereby rotation of said threaded means providessaid movement in a plane transverse to said longitudinal axis.

6. Apparatus according to claim 1 wherein the frequency of said meansfor magnetizing is at least three times the frequency of the magneticleakage flux from said flaw.

7. Magnetic detection apparatus according to claim 6 wherein the meansfor magnetizing provides a continuous alternating magnetic flux between50 and 1000 cycles per second.

1. Magnetic detection apparatus for detecting flaws in a material,comprising; means for magnetizing the surface of a material ofsubstantially circular cross section with an alternating magnetic flux,said flux being directed transverse to the longitudinal axis of thematerial, means for spirally rotating the material along itslongitudinal axis to expose the entire surface of the material to themagnetizing means, said means for magnetizing the surface causing saidmagnetic field to have a zero component in a null plane transverse tosaid field direction and parallel to the longitudinal axis of thematerial, a pair of Hall generator elements for detecting the leakageflux emanating from a flaw in said material, said Hall generatorelements lying adjacent the material surface in a plane parallel to thedirection of the magnetic field with the centers of said Hall elementslying on an axis which intersects a line parallel to the longitudinalaxis of the material at an angle of substantially 5*, means foradjusting the Hall elements in the direction of the magnetic field sothat they are symmetrically mounted one on each side of the null plane,means for detecting the signal outputs from each of said Hall generatingelements being differentially connected to the detecting means toeliminate noise and enhance the signal-tonoise ratio.
 2. Magneticdetection apparatus according to claim further comprising means forapplying a DC bias current to the Hall generator elements and whereinthe DC bias current is of the same magnitude by but different polarityfor each Hall generator.
 3. Magnetic detection apparatus according toclaim 1 further comprising means for mounting each of the Hall generatorelements with respect to the material surface to be independentlymovable in each of three mutually perpendicular directions.
 4. Magneticdetection apparatus according to claim 3 wherein said means for movablymounting said Hall generator elements includes a support structure,means for retaining each of said a Hall generator elements, said meansfor retaining being movable in a plane normal to the surface of saidmaterial, means for moving said retaining means in said normal plane,means for adjusting said means for retaining in a plane parallel to saidlongitudinal axis of said material, and means for moving said means forretaining in a plane transverse to said longitudinal axis.
 5. Apparatusaccording to claim 4 wherein said support structure further includesspaced parallelly extending toothed racks, said means for moving saidretaining means in said normal plane including rotatable means engagingsaid toothed racks, said means for retaining are mounted on threAdedsupport members, said threaded support members are rotatable to providesaid adjustment, and said means for moving said means for retaining insaid transverse plane including threaded means engaging said supportstructure whereby rotation of said threaded means provides said movementin a plane transverse to said longitudinal axis.
 6. Apparatus accordingto claim 1 wherein the frequency of said means for magnetizing is atleast three times the frequency of the magnetic leakage flux from saidflaw.
 7. Magnetic detection apparatus according to claim 6 wherein themeans for magnetizing provides a continuous alternating magnetic fluxbetween 50 and 1000 cycles per second.